code: def forward(self, x):
      layers_ = [x]
  
      for layer in range(len(self.layers)):
          link = self.links[layer]
          tin = []
          for i in link:
              tin.append(layers_[i])
          if len(tin) > 1:
              x = paddle.concat(tin, 1)
          else:
              x = tin[0]
          out = self.layers[layer](x)
          layers_.append(out)
  
      t = len(layers_)
      out_ = []
      for i in range(t):
          if (i == 0 and self.keepBase) or (i == t - 1) or (i % 2 == 1):
              out_.append(layers_[i])
      out = paddle.concat(out_, 1)
  
      return out
  
file_path: PaddleDetection/ppdet/modeling/backbones/hardnet.py
imports: import paddle
  import paddle.nn
  from ppdet.core.workspace import register
  from ..shape_spec import ShapeSpec
label_desc: Use image node_id 4 for calling the HarDBlock obj's underlying member method code with example usage: obj.forward(x) and returns: out